Choosing finishes for exterior wood

Being a natural material, exterior wood degrades quickly under extremes of strong sunlight and high moisture levels. To protect wood outdoors you can either nourish it with oils as you would use a moisturiser on skin, or alternatively envelop the surface in a water resistant plastic film (typically varnish) a bit like wearing a raincoat.

Clear finishes offer less UV protection than coloured finishes as it is the colour in the finish that in the main offers resistance to bleaching. Where timber is dry and has turned grey due to weathering, use OSMO Wood Reviver to restore the colour prior to applying the finish.

Oils

There are several outdoor oils such as Finest Danish Oil, Teak Oil and Decking Oil which are blends of pure oils such as Raw Linseed Oil and Tung Oil. The protection you can expect without retreating will depend on exposure conditions but typically this is around 6 months and up to 12 months in more sheltered conditions. The advantage of oils and modified oils is that unlike varnishes, they do not chip or flake. Oak is best treated with either Raw Linseed Oil diluted with White Spirits, Finest Danish Oil or Pure Tung Oil which gives a paler shade.

Varnishes

Sikkens Cetol Filter 7 is one of the best traditional wood coloured varnish finishes available as it does not significantly hide the grain pattern of the wood and offers excellent weather protection. As Sikkens Filter 7 is not available as a clear product, Sadolin Extra in clear will give good all year weather protection. You should expect around 2-3 years before recoating and up to 5 years in more sheltered conditions.

Modified Oils

There are also products which lie in between a traditional oil and exterior varnish. These include OSMO Country Colour and OSMO Wood Stain & Protector. These products produce a semi-penetrative seal offering approximately twice the life expectancy of a traditional oil finish.